The enzyme YkvM from B. subtilis was identified along with 3 other enzymes (YkvJKL) in a comparative genomics search for enzymes involved in the biosynthesis of queuosine, a 7-deazaguanine modified nucleoside found in Bacteria and Eukarya. YkvM exhibits significant homology to the type I GTP cyclohydrolases, and given that GTP is the precursor to queuosine and that a cyclohydrolase-like reaction was postulated as the initial step in queuosine biosynthesis, YkvM was proposed to be the putative cyclohydrolase enzyme responsible for this reaction. Contrary to the predictions based on sequence analysis, we discovered that the enzyme in fact catalyzes a mechanistically unrelated reaction, the NADPH-dependent reduction of 7-cyano-7-deazaguanine to 7-aminomethyl-7-deazaguanine, a late step in the biosynthesis of queuosine. The reaction is unprecedented in biology, and presented in this talk are in vitro and in vivo studies that demonstrate this catalytic activity, biochemical and bioinformatics analysis that provide insight into the structure and mechanism of this novel family of enzymes, as well as additional comparative genomic analysis and experimental work that has allowed us to identify GTP cyclohydrolase I, the first enzyme of the de novo tetrahydrofolate (THF) and biopterin (BH4) biosynthetic pathways, as the first step in the biosynthesis of 7-deazaguanine modified nucleosides.